Metal alloys are heat treated to produce desirable mechanical properties. Commercially, most aluminum alloys are heated to a temperature of approximately 500.degree. C. (932.degree. F.) and then water quenched. In all of the heat treated alloys, especially those of substantial cross-section, a thermal gradient is produced inside the alloy. The thermal gradient is a result of the surface of the alloy cooling at a much faster rate than the interior of the alloy. As the alloy cools to a uniform temperature, the thermal gradient is removed, but it is replaced by a system of residual stresses.
For applications where the residual stresses must be removed, an uphill quench can be administered to the alloy subsequent to solution heat treatment and traditional quenching. Uphill quenching is a two part thermomechanical process by which the alloy to be stressed-relieved is cooled, preferably to cryogenic temperatures, and then rapidly heated. This reverse thermal cycle produces mechanical plastic deformation which relieves the residual stress.
The ability to relieve residual stress is also a function of the center to surface temperature differential which can be achieved in the alloy part, with the largest temperature differential relieving the greatest percentage of stress. This result is set forth in an article "Uphill Quenching of Aluminum Rebirth of a Little-Known Process" by Tom Croucher in Heat Treating, October 1983, pages 30 through 32. In that article, FIG. 2 demonstrates the varying efficacies of different uphill quench processes that are dependent upon temperature differential and apparently the capacity of heat transfer or heat exchange. The technique of relieving residual stress by the uphill quench process was developed more than 20 years ago. However, its full potential has never been realized due to the physical limitations associated with the conventional steam jet apparatus, which constitutes the present accelerated heating technique in the uphill quenching cycle.
As recorded in the article by Croucher identified above, engineers at Alcoa in the late 1950s identified a desirable uphill quench technique using liquid nitrogen and high velocity steam. The use of high velocity steam presents problems for practical application of uphill quenching of metal alloys of differing size and configuration. The arrangement of steam spray nozzles has been reported to be critical to the effectiveness of this uphill quenching technique. The arrangement is costly and time consuming. In addition, conventional methods only allow for the treatment of one part at a time. Overall, the cost of the uphill quench process is very high as a result of low output rates coupled with the high capital costs associated with installing a high pressure steam boiler and extensive piping. In addition, steam spray nozzles do not heat the part uniformly due to their directional nature. Therefore, upon subsequent machining there is a greater chance for distortion from this form of uphill quenching. Steam spray nozzles are also difficult to align for varying part configuration or dimensions. Finally, the thermal driving force or temperature differential between the center and surface of the part is limited when steam spray nozzles are utilized in light of the physical limitations associated with high pressure steam systems. As a result, the state of the art using liquid nitrogen and high velocity steam is prohibitively expensive and does not lend itself to continuous or multi-part processing, particularly of parts of differing dimension or configuration.
Other discussions of uphill quenching are presented in an article by H. M. Hill, R. S. Barker and L. A. Willey, titled, "The Thermal Mechanical Method for Relieving Residual Quench Stresses in Aluminum Alloys" appearing in Transactions of the American Society for Metals, Volume 52, 1959, as well as an article, "Development of Stress Relief Treatments for High Strength Alumin Alloys", appearing in a quarterly NASA Progress Report, Contract No. NAS8-11091, 1964, Manlabs, Inc.
U.S. Pat. No. 2,949,392 describes an uphill quench technique that preferably uses superheated steam to reduce residual stress in light metals.
Various fluorocarbons are known in the prior art, such as those recited in U.S. Pat. No. 2,459,780, which describes the heat transfer capabilities of fully fluorinated and fully saturated carbon compounds.
These compounds are additionally disclosed in Tetrahedran, 1963, Volume 19, page 1893 through 1899 and in an article entitled "Polycyclic Fluoroaromatic Compounds III", Harrison, et al.
The use of heating solder for vapor phase soldering using fluorocarbons has been disclosed in U.K. patent application No. GP2110204A.
Additional fluorocarbons useful for vapor phase soldering are identified in U.K. patent application No. GP2194231A.
The use of perfluorotetradecahydrophenanthrene has been set forth in U.S. Pat. No. 4,549,686.
The present invention overcomes the drawbacks of the prior art by providing uniform, easily adaptable, continuous, multi-part uphill quenching processes, which provide for large temperature differentials and avoidance of undue mechanical adaptations, or toxic and unstable process material, as set forth below.